1. Field of the Invention
The present invention relates to a hologram recording and reproducing apparatus for recording a hologram in a hologram recording medium and reproducing the information recorded in the hologram.
2. Description of the Related Art
A conventional hologram recording and reproducing apparatus is disclosed in JP-A-H09-305978, for example. The hologram recording and reproducing apparatus splits a laser beam emitted by a laser source into a recording beam and a reference beam, and irradiates a rotating disk, which serves as the hologram recording medium, with the recording beam and the reference beam superposed on each other. The recording beam includes a discretely distributed pixel pattern formed on the wave surface through optical modulation based on the recorded information, and such recording beam interferes with the reference beam so that the hologram is recorded on the rotating disk. When reproducing the recorded information based on the hologram, the reference beam is emitted onto the rotating disk, so that an optical detector for conversion of a reproduction signal detects a reflected beam from the rotating disk. The optical detector detects the reflected beam containing the same pixel pattern as that of the recording beam, to thereby reproduce the recorded information.
The rotating disk, which serves as the hologram recording medium, includes an embossed pit formed in advance along the track. The hologram recording and reproducing apparatus splits a light corresponding to the embossed pit as a servo light out of the reflected beam, and detects the servo light with a tetramerous optical detector for servo control. Thus, the hologram recording and reproducing apparatus controls the track and the focus with the embossed pit, so as to accurately irradiate the rotating disk with the reference beam, to thereby record and reproduce the hologram.                Patent Document 1: JP-A-H09-305978        
In the conventional hologram recording and reproducing apparatus, however, the reference beam may be emitted onto the rotating disk at different angles (tilted), between the irradiation for recording and the irradiation for reproduction. When such a tilt is incurred, the hologram recording and reproducing apparatus cannot secure a sufficient amount of reflected beam for reproduction, and hence fails to obtain the reflected beam with the accurate pixel pattern and to properly reproduce the recorded information, thus resulting in a reproduction error.